Interleukin-28 (IL-28) is a cytokine that comes in 2 isoforms: IL-28A and IL-28B. The two isoforms are 96% homologous. It plays a role in immune defense against viruses, including the induction of the “Antiviral State.” IL-28A has a molecular weight of 22.3 KDa. It exerts its immunomodulatory effect by upregulating MHC class I antigen expression. IL-28A acts as a ligand for the heterodimeric class II cytokine receptor (IL-10RB and IFNLR1) and leads to the activation of the JACK/STAT signaling pathway, resulting in the expression of Interferon stimulated genes, which mediate the Antiviral State. IL-28A also has antitumor activity. Addition of IL-28 to influenza vaccine results in 100% protection from a lethal H1N1 Influenza challenge in a small animal model when it was paired with an Influenza vaccine that protected only 50% of the time without IL-28.